1. Field of the Invention
The present invention relates to the field of thermal exposure, and in particular, determining the thermal history of items and the effects thereof.
2. Description of the Related Art
Consumer goods and products exposed to a variety of temperatures for various periods of time generally suffer degradation in their properties or their suitability for their intended uses or functions. The higher the temperature and the longer the exposure, particularly to higher temperatures, the greater the likelihood of unsatisfactory degradation. This is true for a variety of products, including drugs and other medicines, foods, electronics, munitions and a wide assortment of consumer products, chemicals and compounds—including, for example, batteries, photographic materials, paints, glues, solvents, cleaners, and the like. Effects of temperature exposure on perishable foods and drugs are particularly important.
Unsatisfactory degradation can result in spoiled foods, ineffective medicines, or malfunctioning items, for example. Unfortunately, degradation from thermal exposure can occur under many circumstances. Food items, for example, can spoil at any of various stages from the point of manufacture to use by the consumer. Thermal degradation of products can harm the manufacturer as well as the consumer. A consumer who purchases spoiled food, for example, may avoid that brand in the future. It is therefore important for manufacturers to minimize thermal degradation in their products. As part of this process, it is important for manufacturers to have sufficient knowledge of the types of thermal exposures their products typically experience.
One method manufacturers typically use to help avoid unsatisfactory degradation is date stamping—the labeling of a product with a date, such as the date of manufacture or expiration. Oftentimes, thermal and related stressors, such as direct sunlight, account for the most serious effects of product degradation. Unfortunately, these effects often vary considerably at different times of the year, or even at different times of the same month, and in different geographic locations. Date stamping cannot accurately assess the degree or rate of degradation. Indeed, date stamping can indicate that a perfectly good product should be discarded or that a spoiled product is still good. Accordingly, date stamping is ineffective at accurately assessing the remaining lifetime of an item.
Another drawback to date stamping is that it does not provide sufficient information to determine the cause or responsible party of a spoiled or otherwise degraded product. Because thermal exposures occur at a wide variety of different times and in different circumstances, it is often difficult to assess the cause of product degradation. A food product, for example, may spoil prematurely because it sat too long or at too high of a temperature in either a warehouse, in a distributor's truck, on the retailer's shelf, or even in the consumer's pantry. It is desirable to determine when and where the degradation occurred. By way of example, a manufacturer should not be held liable for product degradation that occurred while it sat in a distributor's warehouse.
A number of efforts have been made to account for the effect of high temperatures and to assess whether such temperatures have yet been reached. These techniques offer a sensor that acts like a fuse, indicating if it has been exposed to a specific temperature. Further efforts have resulted in the development and use of a variety of other temperature sensors, such as Time Temperature Indicators. These sensors are sensitive to both time and temperature. The sensor typically may be visually inspected to determine whether a product has yet degraded past a certain point. This helps each party transferring possession of an item to obtain better information, upon inspection, whether it is still good and thus helps determine the responsible party for spoiled goods, for example.
While such techniques may be useful for indicating that an item was exposed to a specific temperature, they generally do not reveal by how much the specific temperature was exceeded or for how long the item was exposed to any given temperature—both of which can be important factors in determining the expected remaining useful lifetime of the item. Indeed, in some cases, the time of exposure at a specific temperature can be as important or an even more important factor in determining the useful lifetime of an item as the specific temperature of exposure itself. Such devices are typically not very precise, and like date stamping, may erroneously indicate that a good product is spoiled or that a spoiled product is good.
Other sensors have been developed in an effort to provide information on the length of exposure at a particular temperature. These inventions disclose a sensor that indicates exposure to a specific temperature for a pre-determined length of time. However, these techniques provide little, if any, information about intermediate temperature exposure once the item has been exposed to higher temperatures.
All of these sensors, while much more useful than date stamping, have some disadvantages. One disadvantage, for example, is that they are not re-usable and a new sensor must be used for each application. These sensors rely on an irreversible process to indicate changes in time or temperature. With large shipments, the cost of using a new sensor for each item can be considerable. In addition, they do not provide information as to when a specific temperature exposure occurred; rather, they may be capable only of providing information as to whether a specific temperature exposure occurred. That is, there is no way to determine when a thermal exposure occurred once a sensor indicates a pre-determined thermal exposure has occurred.
Another disadvantage is that there is no way to determine the sequence in which different thermal exposures occurred. This information, if available, would allow one to determine who was responsible for a given thermal exposure. Such information would also provide a more accurate determination of the expected remaining lifetime of an item. This is because a product may last significantly longer if it was exposed to a specific temperature recently than it would if it was exposed to the same temperature less recently.
Another important disadvantage of these sensors is that they generally cannot be used interchangeably with different items. Because these sensors respond directly to thermal exposures themselves, it is difficult to tailor the indicator to a specific item or product. In other words, the sensor responds to a predetermined set of thermal exposures and therefore cannot be used interchangeably with different items. For example, a sensor that indicates one item is no longer fresh after being exposed to temperature in excess of 100° F. would not be appropriate for use with an item that maintains its freshness until exposed to temperatures in excess of 120° F. the sensor must be sensitive to the same thermal stressors as the item of interest, which makes the design of such sensors difficult for a wide variety of manufactured items.
Accordingly, there is a need for an improved method and sensor for accurately obtaining the thermal history of an item. In particular, there is a need for determining when an item has been exposed to specific temperatures, thereby providing information as to how long such exposures occurred, in what order they occurred, who was responsible for such exposures, and the expected remaining lifetime of an item or, if the item is no longer suitable, the exposures which caused its degradation and the times such exposures occurred.